This report describes studies on the mechanisms of phosphate and calcium homeostasis relevant to changes in mineral metabolism during the aging process. The findings relate to investigations on: 1. Action of parathyroid hormone (PTH) on Na-Ca exchange in isolated renal cells. 2. iPTH increases in the aged female rat. 3. Mechanism of transepithelial phosphate transport in the proximal tubule. 4. Mechanism of the phosphaturia and hypophosphatemia in the aging rats. 5. Age-dependent alteration in the synthesis of the Vitamin D hormone. 6. Regulation of intestinal calcium absorption by the Vitamin D hormone. 7. Studies of the effect of 1,25-(OH)2D3 on renal calcium transport. 8. Cell culture systems for the study of the cellular mechanism for hormonal regulation of phosphate transport.